The presence of sulfur in industrial gases causes significant environmental problems, and therefore, strict requirements are in place to remove sulfur from gas streams, in particular in petroleum refinery and natural gas plants but also in biogas plants, from H2S scrubbers, etc. A widespread method for desulfurization of sulfur-containing gas streams, in particular from gas streams in petroleum refineries and natural gas plants is the Claus process. The Claus process is long-known and operates in two major process steps. The first process step is carried out by a so-called Claus furnace. In this process step hydrogen sulfide is converted to elemental sulfur and sulfur dioxide at temperatures of approximately 1100 to 1300° C. by the combustion of about one third of the hydrogen sulfide in the gas stream. The so obtained sulfur dioxide reacts with hydrogen sulfide in the furnace to elemental sulfur. Thus, in this first step of the Claus process ca 60 to 70% of the H2S in the feed gas are converted.
To achieve higher sulfur recovery rates two to three catalytic steps follow where the Claus reaction2H2S+SO2=3/xSx+H2Ocontinues. From these steps a gas mixture results which is called the Claus tail gas. The Claus tail gas is usually subjected to further treatment in order to reduce the sulfur content of the gas to an even higher degree. For such further treatment of the Claus tail gas several methods are known, e.g. the subdew point methods, the recycle methods and the direct oxidation methods.
A recent development for the catalytic treatment of the gas mixture obtained in a Claus furnace is the so called “Clinsulf®” process. An overview over the Claus process using the Clinsulf process is provided in the article by M. Heisel and F. Marold in “Linde, Berichte aus Technik and Wissenschaft, 62/1988, pages 33 to 38”. The Clinsulf process is a catalytic process using an internally cooled reactor which is constructed in two sections. The inlet section contains a non-cooled adiabatic bed which allows the reaction temperature to rise quickly and thus increase the speed of reaction. The second section of this reactor comprises a catalytic bed containing a coiled heat exchanger which provides efficient cooling enabling the reactor outlet temperature to be lowered close to the sulfur dew point.
The Clinsulf process has also been adapted to work as a direct oxidation process by introducing an oxygen containing gas into the Clinsulf reactor. This process is mainly used to treat Claus tail gas but it has also been suggested e.g. for biogas desulfurization without the use of a Claus furnace, and here it can be referred e.g. to the article of M. P. Heisel, F. J. Marold and M. Gwinner in “Linde, Reports on Science and Technology, 53/1994, pages 15 to 19”.
The known Clinsulf® reactors contain coiled heat exchangers in the second section of the reactor. This is disadvantageous because such heat exchangers are difficult to manufacture and are thus very expensive. This has prevented so far a wide use of the Clinsulf® process but in particular of the Clinsulf DO® process which is the application of Clinsulf® as a Claus tailgas treatment or in biogas desulfurization. Clinsulf® DO was considered as being (economically) inferior to other processes such as the liquid redox process.
An improvement of the Clinsulf process but not of the Clinsulf DO process is the Clinsulf process using two “Clinsulf” reactors, i.e. two reactors having the inlet section with a non-cooled adiabatic bed and a second section with a cooled catalytic bed. The Clinsulf process and the Clinsulf reactor are also disclosed e.g. in DE 44 09 203. Other recent methods for desulfurization of gases essentially using the Claus process are disclosed e.g. in WO 2010/040495 or WO 2011/005638.
While the Claus process is very widely used in industry, there are several situations where the Claus process is not a suitable option. First of all, a Claus process requires a significant investment and is generally designed to process significant amounts of sulfur-containing gas. For small operations, e.g. small natural gas sources, the installation of a Claus process is usually not economical. Furthermore, the Claus process cannot be used for the desulfurization of gases from chemical plants, such as the desulfurization of hydrogen gas that has been used for the hydrogenation of sulfur-containing gases and is thus contaminated by hydrogen sulfide. Such gases cannot be desulfurized by the Claus process, because such gases would react, essentially be burned in the Claus furnace. The Clinsulf DO process with one reactor has been suggested for the desulfurization of biogas, but it was generally accepted that this process could not be used with gases containing hydrogen, because the hydrogen was believed to react with the catalyst contained in the direct oxidation reactor. The Clinsulf process using two reactors was thus never proposed for anything but as part of the Claus process using a Claus furnace.
For the desulfurization of lean H2S gases (i.e. gases which do not result in a stable flame in a Claus furnace) other methods are used in industry, such as a liquid redox process. In this process hydrogen sulfide is oxidized in an aqueous system at a temperature of about 50° C. using a suitable catalyst, generally a chelated iron catalyst. While the liquid redox process is very efficient in purification of gases containing hydrogen sulfide, the operational availability of this method is usually not higher than about 80% per year, because blocking of parts of the apparatus and ducts is inherent to the system. This results from the fact that three phases are necessarily prevalent in the system: The feed gas and the oxidation air are gaseous, the solvent is liquid, the sulfur produced solid. Further problems are foaming which requires the use of anti-foam agents. Anti-foam agents in the wash solution optimized for foam reduction in the re-oxidation vessel lead to foaming at other stages of the process, e.g. the scrubbing tower. Another problem is that the sulfur obtained with the liquid redox process is very often discolored, sometimes even black. Discolored sulfur cannot be sold so that revenues from the process are low or even negative because the sulfur obtained has to be disposed off, which costs additional money. Furthermore, the process is quite expensive due to chemical consumption in particular of the chelating agent.
Thus, there is a need in industry for a reliable process with a high operational availability and cheap in operation that can be used for the desulfurization of gases, where a Claus process is not economical or cannot be used for technical or chemical reasons. The process should provide a very high desulfurization efficiency of more than 99%.